1. Field of the Invention
The present invention relates to a moving body provided with a mechanism for exerting a preload on a bearing against a guide shaft, and more particularly to an optical pickup apparatus with an objective lens mounted therein.
2. Related Background Art
Conventional optical pickup apparatus are provided with a preloading mechanism for giving a preload on a support portion (bearing portion) against a guide rail as shown in FIG. 1 to FIG. 4. In the apparatus, rollers 102 as a bearing rotationally supported on a carriage 106 are pressed against a reference shaft 101 with circumferential surfaces thereof being in contact with the reference shaft 101 in cooperation with a preloading roller 104 and a fixed roller 103 also as a bearing with circumferential surfaces thereof being in contact with a support shaft 110 set in parallel with the reference shaft 101, thereby restricting rotation of carriage 106 about the reference shaft.
A preloading plate spring 105 for exerting the above pressing force on the preloading roller 104 is a spring formed of a single spring member in an appropriate shape (in an approximately "V" shape in this example), which is arranged to be deformed in the direction of arrow B in FIG. 2 to generate a preload when the carriage 106 is set between the reference shaft 101 and the support shaft 110 in incorporating the base (not shown) of the apparatus. Since this prior art example employs no mechanism or structure for adjusting the preload force, the preload force is determined by a relative position between the shafts and an amount of deformation of the plate spring. Since machining and assembling of those members involved considerable errors, the spring constant of the plate spring has generally been set to be small, particularly in order to absorb the above errors by deformation of the plate spring.
For pursuing high-speed accessibility of optical disk apparatus, the optical pickup apparatus, however, needs to have a size-decreased moving part and vibration characteristics durable for high-speed operation. In order to achieve this object, the apparatus needs to satisfy such a reciprocal relation that moving resistance of the guide portion should be decreased in access whereas the support stiffness by the plate spring should be increased. Namely, the rolling resistance of the guide rollers should be decreased so as to increase a utilization factor of a motor drive force as a propulsive force of the carriage and at the same time, the support stiffness of the carriage should be increased so as to be ready for high-speed operation in a widened control frequency band. When the support stiffness is increased or when the spring constant is increased, there is a possibility that considerable errors are made in machining and assembling of the shafts and plate spring to cause great differences of the preload force, to reduce an elastic deformation range, or to cause plastic deformation of the plate spring in assembling.